Improve Sectioning and ToC

[stefanodavid]

We need to reduce the length of the docs' table of contents, because with the introduction of the Quickstart section (see #250) it is growing too large. To do so, we:

• group sections Introduction & Project Overview, Getting involved, and Accessing the Open Data Hub in one section
• shorten descriptions and content where possible
• (to be checked) reorganise content
• move some sections to appendices.

[mrabans]

done